One of the two commercial methods for generation of pure silicon is the “Degussa method,” which is based on monosilane as educt. The gaseous monosilane, with or without mixing with additional inert gases, is precipitated as silicon at temperatures above 1000° C. The reason for the high temperatures is chiefly to obtain a massive, polycrystalline silicon which can be easily processed to obtain higher purity, e.g., by melting. The thermal decomposition of monosilane into products containing more or less amounts of hydrogen, however, begins at significantly lower temperatures.
References in the literature show that at pressures between 0.13 and 6.61 MPa and temperatures of about 400-600° C., agglomerated spherical particles are produced with an average diameter of 3 μm. Other sources specify the beginning of the decomposition at 450° C. and reported a 75% decomposition at 500° C. and then complete conversion at 700° C. However, lower reaction temperatures resulted in fine-particulate or even amorphous reaction products, which were also observed as byproducts of the Degussa method. Methods which produce fine-particulate silicon condensing from the gas phase as their primary product require an additional processing of the powder, in order ultimately to allow processing into massive silicon.
Relatively little is known about reactions of monosilane with water. From the literature it is known that neutral, liquid water is essentially inert with respect to monosilane, since water can be used to wash out trace gases from monosilane. Likewise, monosilane contaminated with hydrogen also occurs in technical processes.
On the other hand, it is known that Si—H-bonds are not stable in basic solutions, since Si has a high oxophilia. Even the alkali content of glass is sufficient to initiate the decomposition. Likewise, decomposition occurs in acid solution, albeit at a slower rate than in the alkaline pH-range.
From semiconductor engineering it is known that liquid water reacts with silicon surfaces and leaves oxide layers on the order of 1 nm. Likewise, SiO2-layers can be produced under low pressure in CVD processes from various substituted silanes or from Si—H-compounds mixed with hydrogen.
Experiments with mixtures of salts which contain free x-ray amorphic silicon, produced from SiCl4+4 Na->Siam+4 NaCl in nonpolar organic solvents show that inert gas saturated with hydrogen does not have an oxidative effect at room temperature, but that at 400° C. more than 70% reaction is achieved after about 3 hr, and at 550° C. more than 75% has reacted.
The alkaline conversion of silanes for generation of hydrogen is known from JP 59045901 A. This method has the disadvantage that lye is used, that excess water is produced and thus a high mass transport is present.